Suppressor Engineering 101
Posted: Fri Oct 29, 2010 8:51 pm
Suppressor Engineering 101
Disclaimer:
Although care has been taken in preparing the information contained in this document, Mongo does not and cannot guarantee the accuracy thereof. Anyone using the information does so at their own risk and shall be deemed to indemnify Mongo from any and all injury or damage arising from such use. The information contained in these posts is not to be used as a substitute for sound engineering principles and practices. These posts are intended as general guidelines for the design of suppressors but further more detailed engineering is required to adequately determine the safety of a design. Posted documents are provided for informational purposes only. Information in these posts is subject to change without prior notice. ALL NFA AND STATE LAWS APPLY.
Intent:
The intent of these posts is to give a potential designer of a suppressor some idea what is involved in the proper design of a suppressor. These posts will not and cannot cover every possible design configuration that are possible but will give basic guidelines on the most typical methods of construction currently used in the suppressor industry. I will answer general design questions within some limits but I will not answer specific questions on a design unless you pay me for my work. I am employed as an engineer for a living and therefore if you want use of my knowledge you will need to hire me. I will also not address baffle design from the standpoint of gas flow but only from a structural standpoint. It is up to the designer to create the desired gas flow characteristics to maximize the suppression effect.
Definitions:
Here are some of the terms I will be using and their definitions.
Tensile Stress = Tensile stress (also referred to as normal stress or tension) is the stress state leading to expansion; that is, the tensile stress may be increased until the reach of tensile strength, namely the limit state of stress.
Compressive Stress = A stress that causes an elastic body to deform (shorten) in the direction of the applied load.
Hoop Stress = Circumferential stress in a cylindrically shaped part as a result of internal or external pressure.
Safety Factor or Factor of Safety = the ratio of the breaking stress of a structure to the estimated maximum stress in ordinary use.
Buckling = Failure mode characterized by a sudden failure of a structural member subjected to high compressive stresses.
Joint Efficiency (JE) = The ratio of the strength of a joint to the strength of the base metal, expressed in percent.
ASME = American Society of Mechanical Engineering.
Defining the Design Parameters:
Before you begin your design you must define all the parameters that you can. This will narrow the number of design options that can be made and dictate some other options.
You will need to define the following items:
Caliber & Cartridge: Self explanatory, what caliber/cartridge are you going to design the suppressor for.
Design Temperature: This can vary greatly depending on the caliber/cartridge, barrel length, and rate of fire.
Tube Diameter: Self explanatory, though once you begin the design you may decide to change the diameter initially specified.
Mount Design: Self explanatory, this will be up to the designer to determine how the suppressor will be mount on the firearm.
Bore: The diameter of the bore for the suppressor, size of hole through the baffles will need to be determined. The bore on some designs can be tapered to increase in size as the baffles get farther from the muzzle of the firearm. Typically, bore diameter is a constant in most suppressor designs. How do you determine what bore you should use? There are several factors that affect the selection of the bore diameter. The closer the bore diameter is to the outside diameter of the projectile the typically quieter the suppressor will be. The problem with very tight bores is the chance of baffle strike increase. The run out of the suppressor will also effects the required bore diameter. As a suppressor length gets longer, the run out will also increase and should be allowed for in the design. The mounting system affects the run out since all mounting systems have tolerances. The bore therefore should be selected with these factors in mind.
Suppressor Shell Type: If you are going to use tubing as the material for the suppressor shell then you need to select the type. Depending on the material you are using, tubing can be manufactured many different ways. The essential difference between them is if there is a longitudinal seam in the tubing. Seamless obviously is made without a seam where as seamed tubing is welded using various methods. Seamless tubing has a joint efficiency of 1.0 or 100%. Welded tubing can vary depending on the type of weld and the inspection level. EFW (Electric Fusion Welded) or ERW (Electric Resistance Welded) are two methods that are commonly used to make seamed tubing. The joint efficiency can be as low as 0.70 or 70% and as high as 100% depending on inspection levels. Lower the JE the thicker the tubing will be needed to hold the pressure. When ordering the tubing you need to know what the inspection level is if you want to have the best JE. Typical inspection would be full radiography of the weld seam for defects. Remember all tubing has as far as wall thickness, ID and OD. Tolerances can be as high as 12.5% of the dimensions. When doing any calculations for pressure retaining capability of the tubing, the wall thickness tolerance needs to be deducted. DOM (drawn over mandrill) tubing will have the tightest tolerance for the ID and is usually specified in construction of suppressors. Remember not all DOM tubing is seamless. Calculations to determine the hoop stress of tubing under internal and external pressure can be found under the ASME Sec. VIII Division 1 or 2 Boiler Pressure Vessel Code. The BPVC has equations for thin walled pressure vessels of various shapes as well as suggested JE for different types of welds.
Safety Factor: The safety factor that you decide on will be used in the calculations of the various components. ASME suggests a safety factor of 3 to 4 for pressure vessel in various services. Obviously the BPVC was not written for designing suppressors but it at least gives some equations that a lay man can use to determine some thicknesses required for pressures. The BPVC is very complex so it would behoove the designer to hire expert help or become familiar enough with the BPVC that they feel confident in its use.
Disclaimer:
Although care has been taken in preparing the information contained in this document, Mongo does not and cannot guarantee the accuracy thereof. Anyone using the information does so at their own risk and shall be deemed to indemnify Mongo from any and all injury or damage arising from such use. The information contained in these posts is not to be used as a substitute for sound engineering principles and practices. These posts are intended as general guidelines for the design of suppressors but further more detailed engineering is required to adequately determine the safety of a design. Posted documents are provided for informational purposes only. Information in these posts is subject to change without prior notice. ALL NFA AND STATE LAWS APPLY.
Intent:
The intent of these posts is to give a potential designer of a suppressor some idea what is involved in the proper design of a suppressor. These posts will not and cannot cover every possible design configuration that are possible but will give basic guidelines on the most typical methods of construction currently used in the suppressor industry. I will answer general design questions within some limits but I will not answer specific questions on a design unless you pay me for my work. I am employed as an engineer for a living and therefore if you want use of my knowledge you will need to hire me. I will also not address baffle design from the standpoint of gas flow but only from a structural standpoint. It is up to the designer to create the desired gas flow characteristics to maximize the suppression effect.
Definitions:
Here are some of the terms I will be using and their definitions.
Tensile Stress = Tensile stress (also referred to as normal stress or tension) is the stress state leading to expansion; that is, the tensile stress may be increased until the reach of tensile strength, namely the limit state of stress.
Compressive Stress = A stress that causes an elastic body to deform (shorten) in the direction of the applied load.
Hoop Stress = Circumferential stress in a cylindrically shaped part as a result of internal or external pressure.
Safety Factor or Factor of Safety = the ratio of the breaking stress of a structure to the estimated maximum stress in ordinary use.
Buckling = Failure mode characterized by a sudden failure of a structural member subjected to high compressive stresses.
Joint Efficiency (JE) = The ratio of the strength of a joint to the strength of the base metal, expressed in percent.
ASME = American Society of Mechanical Engineering.
Defining the Design Parameters:
Before you begin your design you must define all the parameters that you can. This will narrow the number of design options that can be made and dictate some other options.
You will need to define the following items:
Caliber & Cartridge: Self explanatory, what caliber/cartridge are you going to design the suppressor for.
Design Temperature: This can vary greatly depending on the caliber/cartridge, barrel length, and rate of fire.
Tube Diameter: Self explanatory, though once you begin the design you may decide to change the diameter initially specified.
Mount Design: Self explanatory, this will be up to the designer to determine how the suppressor will be mount on the firearm.
Bore: The diameter of the bore for the suppressor, size of hole through the baffles will need to be determined. The bore on some designs can be tapered to increase in size as the baffles get farther from the muzzle of the firearm. Typically, bore diameter is a constant in most suppressor designs. How do you determine what bore you should use? There are several factors that affect the selection of the bore diameter. The closer the bore diameter is to the outside diameter of the projectile the typically quieter the suppressor will be. The problem with very tight bores is the chance of baffle strike increase. The run out of the suppressor will also effects the required bore diameter. As a suppressor length gets longer, the run out will also increase and should be allowed for in the design. The mounting system affects the run out since all mounting systems have tolerances. The bore therefore should be selected with these factors in mind.
Suppressor Shell Type: If you are going to use tubing as the material for the suppressor shell then you need to select the type. Depending on the material you are using, tubing can be manufactured many different ways. The essential difference between them is if there is a longitudinal seam in the tubing. Seamless obviously is made without a seam where as seamed tubing is welded using various methods. Seamless tubing has a joint efficiency of 1.0 or 100%. Welded tubing can vary depending on the type of weld and the inspection level. EFW (Electric Fusion Welded) or ERW (Electric Resistance Welded) are two methods that are commonly used to make seamed tubing. The joint efficiency can be as low as 0.70 or 70% and as high as 100% depending on inspection levels. Lower the JE the thicker the tubing will be needed to hold the pressure. When ordering the tubing you need to know what the inspection level is if you want to have the best JE. Typical inspection would be full radiography of the weld seam for defects. Remember all tubing has as far as wall thickness, ID and OD. Tolerances can be as high as 12.5% of the dimensions. When doing any calculations for pressure retaining capability of the tubing, the wall thickness tolerance needs to be deducted. DOM (drawn over mandrill) tubing will have the tightest tolerance for the ID and is usually specified in construction of suppressors. Remember not all DOM tubing is seamless. Calculations to determine the hoop stress of tubing under internal and external pressure can be found under the ASME Sec. VIII Division 1 or 2 Boiler Pressure Vessel Code. The BPVC has equations for thin walled pressure vessels of various shapes as well as suggested JE for different types of welds.
Safety Factor: The safety factor that you decide on will be used in the calculations of the various components. ASME suggests a safety factor of 3 to 4 for pressure vessel in various services. Obviously the BPVC was not written for designing suppressors but it at least gives some equations that a lay man can use to determine some thicknesses required for pressures. The BPVC is very complex so it would behoove the designer to hire expert help or become familiar enough with the BPVC that they feel confident in its use.